Fuel cells comprise many types such as phosphoric acid fuel cells, molten carbonate fuel cells, solid electrolyte fuel cells, solid polymer electrolyte fuel cells and alkaline fuel cells. Recently, the ion-exchange membrane fuel cells (solid polymer electrolyte fuel cells) are of great interests, particularly because they operate at a high current density of 3 to 6 A/cm2.
The ion-exchange membrane fuel cells have a polymer ion-exchange membrane comprised of a fluororesin with a sulfonic group (—SO3H) in a side chain. The membrane is disposed between an anode electrode (fuel electrode) and a cathode electrode (air electrode) to form an assembly. Outside the assembly major surfaces are provided current collectors that also function as flow channels for the fuel (H2) and oxidizer (air).
The electrodes are composed of carbon on which a catalyst such as platinum or platinum-ruthenium alloy is supported for promoting the electrode reactions.
The electrode reactions at the anode and the cathode are shown as follows:                Anode reaction: H2→2H++2e−        Cathode reaction: ½O2+2H++2e−→H2O        
The hydrogen ions (Hi) move from the anode to the cathode through the ion-exchange groups in the membrane with water molecules.
The electrode reactions take place at the three-phase (liquid/gas/solid) interface among the electrolytic solution (liquid), air (gas) and catalyst layer (solid). The three-phase (liquid/gas/solid) interface must be adequately formed to prevent the electrode reaction rates from being controlled by the gas diffusion. Therefore, the electrodes are made porous and coated with water-repellent polytetrafluoroethylene (PTFE) resin.
A combination of a diffusion layer and a catalyst layer is called a gas diffusion electrode. Its function is to bring the fuel (H2) and gas (oxygen O2) into even contact with the catalyst layer such as of platinum.
The gas diffusion electrodes (particularly the diffusion layers) have another function of transmitting the electrons created at the catalyst layers, and therefore are required to have conductivity.
Furthermore, the diffusion layers have a function of distributing the gas to the catalyst layer, and therefore include a great number of pores.
In order to meet the need of ensuring the ion conductivity of the ion-exchange membrane and of hydration, humidified fuel (H2) is used. The gas diffusion electrodes have water repellency to prevent the moisture for ion conductivity from clogging the pores in the diffusion layers to block the gas diffusion.
As described above, the gas diffusion electrodes for use in the fuel cells and other electrochemical devices are required to have gas diffusibility, gas permeability, water repellency and conductivity.
The diffusion membranes (layers) are generally produced by coating an electrode material such as a porous carbon fiber membrane or porous carbon paper with a dispersion mixture that contains carbon powder, polytetrafluoroethylene (PTFE) and a dispersion medium, by means of spraying or printing technique.
In such conventional processes, the coating of the porous carbon fiber membrane or porous carbon paper with a dispersion mixture that contains uncalcined polytetrafluoroethylene, conductive carbon and a dispersion medium is followed by repetitive rolling. This requires complex and multiple steps and increases the processing cost.
Moreover, the above process is complicated and has great difficulty in controlling the application of the dispersion mixture so as to achieve uniform properties throughout the resultant diffusion layer. It is also insufficient in terms of mass production capability for the diffusion membranes (layers). Furthermore, the porous carbon fiber membranes or porous carbon papers are expensive to cause cost problems.
To solve such problems, JP-A-2001-85280 (patent publication (i)) owned by the present applicant proposes a production process for polytetrafluoroethylene-containing sheet electrodes and a sheet electrode obtained by the process. This process comprises rolling a rod-shaped preform into a sheet electrode, wherein the preform comprises carbon fine powder, polytetrafluoroethylene and a liquid lubricant and has a specific relation with respect to the maximum length in the compression direction and that in a crosswise direction perpendicular to the compression direction.
The above process is capable of manufacturing uniform sheet electrodes through a simple step and enables cost reduction and mass production.
However, the method of JP-A-2001-85280 (patent publication (i)) using the conventional raw materials, namely, uncalcined polytetrafluoroethylene and conductive carbon results in a diffusion membrane (layer) which has unfavorably high volume resistance and insufficient electron conductivity and gas permeability. This can be rationalized by the excessive binding effect of the uncalcined polytetrafluoroethylene whose intended purpose is to impart water repellency. Furthermore, the method has difficult membrane production and is unsuitable for continuous mass production of membranes.
Under these circumstances, the present inventors earnestly carried out studies of a low cost process for mass production of diffusion membranes that have sufficient water repellency and no regional variation in properties. As a result, they have found that a resin composition containing uncalcined polytetrafluoroethylene, calcined polytetrafluoroethylene and a conductive substance can solve all the aforesaid problems by its capability of giving diffusion layers that have adequate water repellency and no local variation in properties, at reduced cost and with mass producibility. The present invention has been accomplished based on this finding.
JP-A-S52-97133 (patent publication (ii)) discloses a gas diffusion electrode including a sheet of a carbon material that contains a specific amount of carbon fibrils held together with a fluororesin, wherein the fibril lengths and diameters have a specific ratio. It also discloses a production process for gas diffusion electrodes comprising kneading and rolling the carbon material, the fluororesin and an auxiliary into a sheet and bonding the sheet with a catalyst layer mainly composed of an electrode catalyst and a binder, wherein the auxiliary is removed before or after the catalyst layer is bonded.
The gas diffusion electrode as described in the above patent publication is relatively thin and satisfactory in mechanical strength as mentioned also in JP-B-S63-19979 (patent publication (iv)), but the gas permeability thereof is insufficient.
JP-A-S58-165254 (patent publication (iii)) discloses a process for producing fuel-cell gas diffusion electrodes that comprises forced filling of carbon powder, a catalyst and ethylene tetrachloride resin into pores of a porous fiber membrane by means of suction force from below one side of the membrane.
JP-B-S63-19979 (patent publication (iv)) discloses a gas diffusion electrode material that is a fine porous network structure. This material comprises a number of polytetrafluoroethylene resin minute nodes containing conductive substance powder and a number of polytetrafluoroethylene resin fibrils containing no conductive substance powder that extend from each node to form three-dimensionally linked nodes, wherein all the minute nodes contact with each other or are in series at parts thereof.
JP-B-H01-12838 (patent publication (v)) discloses a gas and liquid permeable electrode material resulting from the bonding of a liquid permeable membrane that contains conductive substance powder in minute nodes and a conductive porous substrate of specific flexural strength. The liquid permeable membrane is a porous polytetrafluoroethylene resin membrane that has a number of minute nodes dimensionally linked through many fibrils with formation of a spiderweb network among the minute nodes. The minute nodes contact with each other or are in series at parts thereof.
JP-B-H05-52031 (patent publication (vi)) discloses a gas diffusion electrode material comprising:
a plurality of conductive powder-containing layers that are constituted of a number of polytetrafluoroethylene resin minute nodes containing conductive substance powder and substantially connected with each other, and a number of polytetrafluoroethylene resin fibrils that extend from each node and tridimensionally link the nodes; and
a layer that is disposed between the plurality of layers and is constituted of a number of polytetrafluoroethylene resin minute nodes containing no conductive powder and a number of polytetrafluoroethylene resin fibrils extending from each node to tridimensionally link the nodes. These layers are directly engaged by pressure whereby the conductive powder-containing minute nodes are pressed and dispersed into among the fibrils that link the minute nodes containing no conductive substance powder. Consequently, electric conduction is achieved as a result of partial contact between the conductive powder-containing minute nodes or by the jumping effect.
However, these patent publications (ii) to (vi) provide gas diffusion electrode materials that are unsatisfactory in any of gas permeability, water repellency, mass producibility, production costs, conductivity and uniformity.
The present invention aims to solve the aforesaid problems related to the background art. It is an object of the invention to provide a low cost diffusion membrane that employs neither the expensive porous carbon fiber membrane nor porous carbon paper and has sufficient water repellency, uniform properties throughout the layer and suitable mass producibility.
It is another object of the invention to provide an electrode having a diffusion membrane (layer) with the above properties.
It is a further object of the invention to provide an efficient and low cost process for producing diffusion membranes having the above properties.